This invention relates to methods of generating electric signals having approximately sinusoidal waveforms and to generators for supplying such signals.
More particularly, but not exclusively, the invention is concerned with the generation of electric signals of the kind having an approximately sinusoidal waveform and selectively one or other of two fundamental frequencies.
It is well known to signal binary data by the technique of frequency shift keying. One example of this is in the Viewdata system currently adopted by the British Post Office, data being signalled over a telephone line in one direction at a rate of 75 baud by selectively transmitting oscillations of either 390 or 450 hertz and in the opposite direction at a rate of 1200 baud using oscillations of 1300 and 2100 hertz. Hitherto in generating such oscillations it has been usual for the frequency to be changed abruptly from one value to the other, in response to the data to be signalled, but doing so results in the production of unwanted transients which have to be filtered out before the frequency shift signal is passed to line. Filters for that purpose tend to be expensive and one object of the invention is to generate the frequency shift signal in a novel way so as to reduce the amount of filtering required.